Argon stirring of molten steel for temperature homogenization is well known in the art. In such processes, low volumes of an inert gas, such as argon, typically 0.03 to 0.06 m.sup.3 /ton, are injected into a ladle of steel to cool the steel to a uniform and suitable temperature for continuous casting. A common technique is to immerse a lance or a hollow dummy stopper rod through which argon gas is admitted for a period of three to five minutes at about 10 scfm (0.3 m.sup.3 /min.). It is generally recognized that uncontrolled argon stirring may have a deleterious effect in that excessive agitation may excessively expose the steel to the atmosphere or oxidizing slag to reduce the steel's cleanliness.
Argon degassing is another well known procedure wherein generally large amounts of an inert gas, such as argon, i.e. ten to twenty times the amount used in stirring, are blown through a molten steel to reduce the oxygen and hydrogen content. These procedures usually require rather sophisticated equipment, and treatment costs are relatively high.
Argon trim stations have been reported where final deoxidant or alloy additions are made in the ladle during or after argon stirring. The stirring action is usually very turbulent. The argon treatment is used to assist in mixing the deoxidant or alloy addition, thus achieving better recovery of the added elements, and is intended to produce chemical and temperature homogeneity.
While uncontrolled argon injections may adversely affect the steel's cleanliness, it has been recognized that controlled argon injection into molten steel may serve to remove some of the non-metallic inclusions, such as oxides and sulfides. Such a cleansing action, however, is minimal, and in no way comparable to the various vacuum degassing processes. That is to say, that while low volume argon flushing practices have been developed to mix a molten steel, the degree of cleanliness achieved is in no way comparable to that effected by conventional vacuum degassing practices, such as DH-degassing. For example, one study has shown that for a particular electric furnace steel grade containing 0.21 to 0.30% carbon, the uncleansed product contained an average oxygen content of 121 ppm. The product oxygen content was reduced to 114 ppm with conventional argon stirring, while the product oxygen for DH-degassed samples averaged 69 ppm.
It has been unfortunate that argon flushing practices cannot be substituted for vacuum degassing because, as the demand for high quality steels increases, many steel mills are experiencing a shortage of vacuum degassing capacity.